The present invention relates to reflectometers for measuring the reflectance of a test material. More particularly, the present invention relates to reflectometers that measure the angular distribution pattern of light reflecting off the test material.
In a number of disciplines such as remote sensing, computer graphics, and aircraft signature prediction, the reflection properties of materials must be precisely determined. In particular, the bidirectional reflectance distribution function (BRDF) defines the distribution of the reflected light rays that are associated with each possible incident direction of light. For a particular wavelength of light, the BRDF is a function of four variables. Two of the variables define the direction of incident light. The remaining two variables define the direction of reflected light. For isotropic materials, the BRDF is independent of the azimuth orientation of the sample. Therefore, for isotropic materials, only three angles are needed to describe the BRDF. Anisotropic materials, however, require the four variables to describe the BRDF and are much more difficult to characterize.
In practice, the BRDF of anisotropic materials is extremely difficult to measure with any degree of completeness due to the large number of angle combinations for the incident and reflected light. For example, if the BRDF measurements were made by moving a light source and a detector in two degree increments, over 65 million separate measurements are required. If each individual measurement could be accomplished in one second, the complete BRDF measurements would take over 2 years.
Surface Optics markets a portable measurement device that operates in the infrared (IR) region. The portable measurement device uses a movable source and detector. Furthermore, in U.S. Pat. No. 5,637,873, which is incorporated by reference, a hand-held instrument uses angular imaging to measure the directional reflectance of materials after they have been applied to a vehicle. This instrument is suitable for verifying compliance of in situ coatings with their reflectance specifications. Both devices, however, do not provide a complete and automated characterization of the BRDF of a material.
A reflectometer according to the invention characterizes the reflectance properties of a test material. The reflectometer includes a radiation subsystem that generates and directs radiation onto a test material at a plurality of incident angles. An elliptical reflector assembly has one or more reflectors with first and second foci. A holder positions the test material at the first foci of the reflectors. One or more lenses are located within a first focal length of the second focus of the reflectors. The lenses receive angular images that are reflected by the reflectors.
According to other features of the invention, the elliptical reflector assembly includes a first reflector having first and second foci and a second reflector having a third and fourth foci. A first lens is located at said second focus of said first reflector. A second lens is located at said fourth focus of said second reflector. The holder positions the test material at the first and third foci of the first and second reflectors.
According to other features of the invention, the holder is rotatable relative to the radiation subsystem. The radiation subsystem includes a housing that is movable relative to the elliptical reflector assembly to alter the incident angle. A focusing mirror is connected to the housing. A slit controls the shape of the radiation that is illuminated by the test material. The slit is movable relative to the housing to keep the shape and size of the illumination spot relatively constant as the housing moves.
According to still other features of the invention, a shutter blocks the radiation when in a closed position and passes the radiation when the shutter is in an open position. Ambient reflection and sample emissions measurements are made when the shutter is in the closed position.
According to still other features of the invention, a first stepper motor adjusts an angular position of the housing relative to the elliptical reflector assembly to adjust an incident angle of the radiation on the test material. A position encoder generates a position signal that is related to the angular position of the housing.
According to still other features of the invention, a second stepper motor adjusts an angular position of the holder. A second position encoder generates a position signal that is related to the angular position of the holder.
In still other features of the invention, a computer is connected to the first and second stepper motors. The computer is also connected to the first and second position encoders. A first imaging assembly receives the first angular image and generates a first angular image signal. A second imaging assembly receives the second angular image and generates a second angular image signal. The computer generates a first difference signal by subtracting an ambient first image signal from the first image signal. The computer generates a second difference signal by subtracting an ambient second image signal from the second image signal. The computer generates a calibrated first product signal by multiplying the first difference signal by a first set of calibration factors. The computer generates a calibrated second product signal by multiplying the second difference signal by a second set of calibration factors. The computer combines the calibrated first difference signal with the calibrated second difference signal to create a hemispherical angular image signal.
Other objects, features and advantages will be apparent from the specification, the drawings and claims.